While Starfleet offers its personnel a wide variety of branches from which to choose, the Command, Operations and Science branches have the most members. Starfleet reassigns some as needed, or as their and aptitudes become clear. Others ask for transfers from branch to branch, but most choose a branch at the start of their careers and stay with it. What follow are short descriptions of these branches as well as some of the positions characters fill. Most of these are bridge positions; many of the other positions which exist throughout a ship can be found under Shipboard Positions.

Command

Command probably sees a higher percentage of people transfer into it than any other branch, but a fair number of officers begin and end their careers in its ranks. Most spaceship and starbase commanders come from this branch, as do many diplomats and instructors. Those in the command branch train to handle a variety of situations, including many types of crisis resolution.

Commanding Officer

Every Starfleet facility has a single commanding officer responsible for the safety of her crew and the success of their missions. She has final accountability for any actions taken by her facility or her crew, and Starfleet goes to great lengths to prepare a CO for the job she must handle, including the interpretation and execution of Starfleet rules and regulations. All commanding officers are rigorously screened and tested, undergoing a battery of psychological, physical and ability exams. A superlative example of a starship CO is Captain Picard of the U.S.S. Enterprise-D, while Commander Benjamin Sisko of Deep Space 9 represents an excellent starbase CO.

Executive Officer

Besides leading a facility when the Commanding Officer is unable to, the XO (sometimes called the first officer) often assists the CO and serves as liaison with the crew. He oversees a ship or base's day-to-day operations and advises the CO on important matters. While Starfleet expects XO's to anticipate and resolve confrontations in a nonviolent manner, it provides them extensive training in battle maneuvers. The role of XO on the U.S.S. Enterprise-D is filled by Commander William Riker.

Operations

The Operations branch covers the widest variety of duties, handling everything from engineering to security to maintenance. It has a large number of departments and subcategories like Advanced Tactical, Environmental Systems and Communications. Talented members of Operations often go into Command, taking control of ships after proving themselves. Sometimes, Command personnel transfer to Operations, as well.

Flight Control Officer

Also known as the Conn, a Flight Control Officer must always be present on the bridge of a starship. She plots courses, supervises the computer's piloting, corrects an flight deviations and pilots the ship manually when needed. On starbases and planetary bases Flight Control Officers have far less to do but are still present. All Starfleet bases have some spaceship capabilities, and their Conn officers pilot these ships as needed. This is often a proving ground for new Conn officers until they earn a berth on a starship. Geordi La Forge once held the Conn on the Enterprise-D before being promoted to full lieutenant and chief engineer.

Chief Engineer

The Chief Engineer is responsible for the condition of all equipment on a Starfleet facility or ship. He oversees maintenance, handles repairs and requisitions needed equipment. He also commands repair teams during crisis situations Unlike most other starship offices, his primary post is not on the bridge but in the engineering section. B'Elanna Torres assumed the duties of chief engineer of the U.S.S. Voyager after joining the Starfleet crew.

Operations Management Officer

The Operations Manager, also known as the Ops, has the primary responsibility of ensuring that ship functions — such as use of the lateral sensor arrays — do not interfere with one another. She must prioritize resources like phaser, technical resources like energy and personnel resources like Away Teams) so that the most critical activities can have every chance of success. She can curtail any shipboard functions if she thinks they will interfere with the ship's current mission or routine operations. Lt. Commander Data performs Ops duties on the Enterprise's bridge.

Mission Operations Officer

The Mission Ops primarily serves as a backup to Ops, but also serves as a starship or base's main link to any Away Teams. He monitors all Away Team communications and advises Ops accordingly.

Environmental Systems Officer

This position, though technically under the chief engineer, mainly works as a deputy Operations Manager, advising the current Ops as to the status of life support. Environmental Services posts often assume critical importance during yellow and red alerts.

Tactical Officer

The Tactical Officer is responsible for handling both internal and external threats, and usually has the additional role of senior tactical officer. He commands security teams throughout the facility or ship and oversees the handling of any prisoners and ship defense. He often has a second tactical officer to assist him and take his post if he is incapacitated. Worf administers security details on the Enterprise-D.

Science

Science handles most of Starfleet's research and development, though Starfleet encourages all its personnel to experiment and make scientific inquiries. Scientists on board Starfleet vessels sometimes see new phenomena before any other people, encountering new forms of matter and energy and making breakthrough discoveries. In the past starships often had science officers to coordinate their experiments and advise the captain on such matters; since then Starfleet has found that a wide variety of experts can provide better advice. Specialists have taken over these roles, and most commanding officers meet with an entire staff to hear all their recommendations. In many cases CO's will have one or two experts who consolidate technical information and give clear and concise advice.

Medical

Starfleet Medical is grouped alongside the Science division. It includes all personnel in Starfleet specializing in the physical treatment and care of lifeforms. Equipped with state-of-the-art equipment, Starfleet's doctors can handle a variety of alien plagues and viruses, treat previously unknown lifeforms and perform valuable medical research.

Chief Medical Officer

A chief medical officer does more than patch up injured crew members. His main function is to ensure that they don't get sick or injured to being with, and to this end monitors their health and conditioning. If necessary, the Chief Medical Officer can remove anyone from duty, even a commanding officer also oversees all sick bay activities. Doctor Beverly Crusher serves as CMO on the U.S.S. Enterprise-D on the majority of its missions.

Counselor

Because of their training in psychology, technically the ship's Counselor is considered part of Starfleet Medical. Found generally on starships which undertake frequent diplomatic missions or have large crews, the counselor is responsible both for advising the commanding officer in dealing with other people and races, and in helping crew members with personal, psychological and emotional problems. Deanna Troi, the Betazoid Counselor of the U.S.S. Enterprise, uses her empathic abilities to aid in her duties. While Starfleet has enjoyed remarkable success with such empathic and telepathic counselors, many counselors without these talents do their jobs admirably.